Hitherto, to turn a strut or a wrist shaft at a distal end portion of a robot, to drive a conveyor and the like, an infinite rotation control, in which each of rotary shafts of the robot and the conveyor performs a turn of not less than 360 degrees, has been employed.
This type of conventional conveying apparatus is disclosed, for example, in Japanese Patent Laid-Open No. 79674/1994. In this conventional apparatus, to achieve the infinite rotation control capable of turning a wrist shaft of a robot in the order of 360 degrees and a predetermined angle positioning control by using the same hardware, the following control operation is carried out. That is, at the time of infinite rotation control, the infinite rotation is conducted by resetting a signal from an encoder mounted on a drive unit such as a motor to turn the wrist shaft each time the wrist shaft make the rotation, and counting up number of rotation every time. On the other hand, at the time of the angle positioning control, the angle positioning is conducted by causing the drive to rotate until a signal from the encoder comes to be a predetermined value, establishing a reference encoder value reset each time of turning as origin. Further, in the disclosed conveying apparatus, by setting a rotation ratio R between the wrist shaft and the drive to be 1:2n (n is an integer), accurate return of the wrist shaft to the origin can be performed without any mechanical gap even after the infinite rotation.
Moreover, The Japanese Patent Laid-Open No. 44076/1998 discloses another conveying apparatus, in which the rotation ratio R between the wrist shaft and the drive can be set arbitrarily to be, for example, N/M. When the drive makes N rotations, a rotation counter counts up M, thereby achieving the infinite rotation. Further a rotational angle of the wrist shaft is calculated based on a rotation amount of the motor drive shaft until there is an increase in M counts.
Furthermore, the Japanese Patent Laid-Open No. 217171/1998 discloses a technology in which a signal from an encoder mounted on a drive for turning a wrist shaft is reset each time it make the rotation, and number of rotation is counted up every time, thereby achieving the infinite rotation. Further, by setting the rotation ratio R between the wrist shaft and the drive to be 1:2n (n is an integer), the drive is rotated in a direction close to an origin, whereby accurate return of the wrist shaft to the origin can be performed in a short time. Furthermore, after setting a current value to a working origin, that is, to a value of rotational angle from the viewpoint of the reference encoder value, the wrist shaft is returned to the origin, thereby solving a disadvantage of the mechanical gap.
In the conventional conveying apparatus described above, utilizing an action that the rotary shaft makes M rotations by deceleration means when the drive makes N rotations, the encoder value is changed when the drive has made N rotations. That is, the encoder value can be changed only in such a limited case that the rotation number M of the rotary shaft and the rotation number N of the drive are respectively integers or values conforming to a resolution of the encoder. However, a problem exists in that setting below the resolution is impossible, and that conduction of conveying work repeatedly brings about an accumulated error, making it impossible to carry out an exact conveying work.
Moreover, in the case of linearly moving any article to be conveyed in conformity with a distance between one step and another such as carried out by a conveyor belt in manufacturing line, the drive for driving the conveyor requires plural times rotation control and positioning control at a predetermined angle. Further in the case that the deceleration means is a belt, or that means for conveying the article to be conveyed is a conveyor belt, it is impossible to indicate the rotation ratio of the deceleration means in the form of an integer. Therefore, a problem exists in that conduction of any conveying work repeatedly brings about an accumulated error, making it impossible to carry out an accurate conveying work.
Furthermore, in the case of repeating plural times linear movement or rotational movement in one direction at all times, when executing a program based on a current value, the current value increases sequentially in order. Therefore, a value indicative of a position command in the program comes to be larger gradually, and thus a problem exits in that the program becomes further complicated.